The invention relates to the field of motion picture film projectors, and in particular to an illumination system suitable for reducing film buckle in a film gate.
Motion picture film projectors have been successfully used in theatres for decades to project high quality imagery. Aside from the development and popularization of very large screen film formats (such as 70 mm) and the associated projection equipment, the industry has made very few design changes to film projectors which actually improve the on screen image quality. Among the various problems which degrade the projected image quality, including the obvious scratches, dirt, and film jump and weave, film buckle is a less obvious, but perhaps the most significant, contributor to quality loss.
The basic phenomenon of film buckle is discussed in the paper xe2x80x9cModulating Air Blast for Reducing Film Bucklexe2x80x9d by W. Borberg in the Journal of the SMPTE, Vol. 59, August 1952. While much of the light incident on the film is transmitted through it, and subsequently imaged to the screen by the projection lens, a portion of this light is absorbed, either by the dyes in the case of color film, or by the silver grains in the case of black and white film. Indeed, the images themselves are stored on the film as spatially varying density patterns, which rely on light absorption, rather than light reflection, to modulate the incident light. However, the absorbed light in turn heats the film, which then being an elastic material, deforms out of the film plane. This thermally induced deformation or pillowing can shift the image in a variable fashion, outside of the designed depth of focus of the projection lens, and degrade the on screen image resolution. Film buckle is further complicated by the action of the shutter, which is typically located between the lamp source and the film gate in most projectors. For example, when the popular two bladed shutters are used, the incident light to the film sees two short pulses of light per film frame. The film buckles during the first illumination period, relaxes some during the intervening dark period, and then buckles or deforms further during the second illumination period.
A variety of methods have been used or considered to alleviate or compensate for the problem of film buckle. In his paper, Borberg describes the results of his experiments to counter the film buckling with either continuous or pulsed inputs of pressurized air. Although the pressurized air undoubtedly cools the film to some extent, the pressurized air provides its greatest contribution to counteract buckle by providing a counteracting force to push against the deforming film surface. Unfortunately, while the pulsed air experiments were successful, the Borberg approach was not adopted by the industry due to the mechanical complications and the noise generated by the pressurized air system.
As another approach, a custom dichroic shutter, with blade areas which block visible radiation while transmitting the infrared has been attempted. The intent was to utilize the plentiful infrared radiation emitted by an unfiltered xenon arc lamp source to heat the film image area during the shuttered or dark periods, such that the thermal load and resulting film buckle would be averaged and less variable within the frame time. However, as color films transmit the greater majority of incident radiation, and absorbs perhaps as little as 5 percent of such light in the near infrared (out to xcx9c2 nm), this method was only marginally effective. Furthermore, as discussed in the Borberg paper, and to a greater extent in another paper, xe2x80x9cProjection Performance of Theatrical Motion Picture Films using Xenon Short Arc Lampsxe2x80x9d, by P. Preo, further heating of the film frame will cause yet further film deformation, and beyond certain thresholds, actual damage such as dye fade, blistering, or scorching.
Likewise, other approaches, such as preheating the film frame image area before projection, so as to reduce the range of deformation which would occur during projection, have been considered, but would only add further thermal loading to the film in a minimally useful manner.
The most effective method employed in the industry to counteract the effects of film buckle is to design the projection lenses with curved object xe2x80x9cplanesxe2x80x9d, such that the region of best focus follows a curve roughly corresponding to the curvature of the film deformation or buckle. While this technique has proven reasonably effective, the curved plane of best focus is static, while film buckle is dynamic, varying not only with the exposure time in the gate, but also with film density, the actual film type or dye layer formulation, and finally with the film base formulation.
Thus, it would be desirable to provide alternate approaches to the problem of film buckle in which the thermal deformation experienced within the film frame image area is actually reduced.
It is an object of the present invention to provide full film width illumination, rather than merely frame width illumination, which reduces film buckle by one-third to one-half, and thereby improves the projected image resolution. Targeted heating of the film can most effectively be applied to the areas between the film edges and frame edges by incident light, rather than some other means, such as a heater plate. Various optical system configurations are possible, based on both traditional and new optical illumination designs, where light can be directed onto the areas between the frame edges and the film edges, without cutting into the efficiency of the light delivered to the screen. This idea could also be used for film scanners.
The on-screen image quality provided by motion picture film projectors is significantly degraded by film buckle; that is warpage or deformation of the film when it is subjected to a heat load by the incident light. This warpage typically causes portions of the film to be deformed sufficiently that some portions of the image are displaced outside the projection lenses"" depth of focus for optimal screen imaging, resulting in a loss in image resolution with field. The present invention corrects these deficiencies.
The invention and its objects and advantages will become apparent in the detailed description of the preferred embodiment presented below.